A known centrifugal separator of this art is indicated in FIGS. 1, 2, 2a and 2b of the present specification.
Generally in a centrifugal separator a feed containing two or more phases is separated into individual phases, e.g. a solid or heavy phase and a liquid phase. A liquid phase may comprise a heavy liquid phase and a light liquid phase. A solid phase may by means of a screw conveyor be transported to an outlet at a front end of the separator while the liquid phase flows to an outlet at the rear end of the separator.
In a centrifugal separator the feed is accelerated to high velocities, the material closest to the circumference of the bowl having the highest velocity and thus the highest kinetic energy. Discharging material from the bowl at a position close to the circumference therefore entails a great loss of energy. To recover this energy two means are provided: The first means is discharging the material from the bowl at a position close to the axis of rotation, and the second means is ejecting the material with a relative velocity in a direction opposite to the direction of rotation; thus the material is discharged with a relatively small absolute velocity.
DE-A-39 04 151 discloses a centrifugal separator, in which the base is provided by a double wall with a ring chamber between the two walls of the double wall. A circumferential wall of the ring chamber, placed between the two walls of the double wall, is provided with two outlet nozzles. A concentric hole is provided in the wall closest to the separation chamber thereby providing the first weir at a first level. Openings are provided in the other wall, said openings being partially covered by adjustable weir members defining overflow outlet openings at a second level above the first level. Thus the overflow outlet openings open into a rear longitudinal area defined by the base, while the outlet nozzles open into another longitudinal area in front of the rear longitudinal area. The outlet nozzles in the circumferential wall are dimensioned to discharge 80-90% of the liquid phase volume during full load operation. At the end of the bowl opposite the base a solid phase outlet is provided at a level between the first and the second level. Hereby is obtained that during start-up the separator may be run at partial load and with a level of liquid inside the bowl rising to the first level i.e. below the level of the solid phase outlet. Thus there is no risk that liquid phase will exit through the solid phase outlet during the start-up. When an amount of solid phase has build-up at the solid phase outlet, thus blocking out the liquid phase from the solid phase outlet, the feed rate may be raised to full load, the level of liquid rising to the second level. This possibility of running the separator with two different levels of liquid inside the bowl is the purpose of the double wall construction. To recover energy the outlet nozzles are directed obliquely opposite the direction of rotation. In this prior art separator a part of the liquid phase is discharged relatively close to the axis of rotation in the area rear of the bowl.
DE-A-31 12 585 discloses a centrifugal separator with a horizontal axis of rotation and a liquid phase outlet at one end. In one embodiment the liquid phase outlet is provided as two successive annular outlet or collecting chambers separated from a separation chamber of the separator and from each other by identical annular discs. Curved outlet pipes with outlet nozzles are connected to circumferential walls of the collecting chambers to discharge liquid in a direction opposite the direction of rotation of the separator. Due to the fact that the annular discs are identical, liquid phase will flow at a first level above the inner annular edge of the first annular disc into the first collection chamber from the separation chamber to be discharged through the curved outlet pipe(s) connected to that collecting chamber. If the first collection chamber runs full due to excessive liquid phase flow, an excess part of the liquid phase will flow across the first collection chamber and into the second collection chamber. At this time the liquid phase in the separation chamber adjacent the first annular disc has risen above the first level to provide a pressure head driving the liquid phase across the first collection chamber.
Discharging material from the bowl at a position close to the axis of rotation is for practical reasons only possible if the material is discharged into the rear area, which is rear of the bowl and defined by the base of the bowl. In some instances it is not possible to discharge a liquid phase into this rear area.